The proposed research concerns the chemical and molecular properties of C3b, the active form of the third complement protein. C3b binds to a variety of receptive surfaces such as cell membranes and inert substrates such as zymosan, isolated bacterial lipopolysacccharide, inulin, Sepharose, and Sephadex. Previous work in this laboratory has demonstrated that human C3b binds to receptive surfaces by a covalent bond which is most likely an ester bond. Because the activation of C3 and the binding of C3b to cell surfaces is a key step in the role that complement plays in the immune system, it is essential to understand the chemical and molecular properties of C3b that permit it to form covalent bonds to receptive surfaces. In the research proposed here we will, 1) identify the amino acid on C3b and the chemical group on the receptive surface that are responsible for the formation of the covalent bond, 2) isolate the peptide containing the labile binding site and determine the amino acid composition and sequence of the peptide, 3) investigate the mechanism for the formation of the ester bond, 4) study the mechanism of the spontaneous cleavage of the covalent bond that leads to the natural release of C3b from receptive surface, and 5) determine whether guinea pig C3b and human C4b bind covalently to receptive surfaces.